Coating ferrous metal sheets with an insulating film



Oct. 11, 1949.

fjgrj.

J. NAGEL ET AL COATING FERROUS METAL SHEETS WITH AN INSULATING FILMFiled April 3, 1946 INVENTORS fiv/z j; A/aya/ and an l/marl PatentedOct. 11, 1949 COATING FERROUS METAL SHEETS WITH AN INSULATING FILM FritzJ. Nagel, Homewood, and Clifford C. Horstman, Sharpsville, Pa.,assignors to westinge house Electric Corporation, East Pittsburgh, Pa.,a corporation of Pennsylvania Application April 3, 1946, Serial No.659,415

4 Claims.

This invention relates to insulation, and in particular to tenaciouslyadherent films possessing high ohmic resistance for magnetic material.

In preparing magnetic materials, such, for example, as silicon iron, forthe building of magnetic cores therefrom, it is necessary that thelaminations be provided with electrical insulation between one anotherin order to minimize eddy currents and to achieve low losses. Theinsulation should preferably be an extremely thin film to provide for ahigh space factor. In many cases, it is further necessary that theinsulating material applied to laminations should withstand the elevatedtemperatures which are encountered in strain annealing cores afterassembly in order to improve their efficiency. Temperatures encounteredin strain annealing occasionally reach 1200 C. and are rarely less than600 C. In addition, it is highly desirable that the insulation for themagnetic sheet material be capable of withstanding bending, scraping andother mechanical abuse or chemical treatment which may be encountered informing the magnetic material to shape and assembling cores therefrom.Magnetic material is usually cut or punched after the insulatingmaterial is applied thereto, and thereafter the cut or punched magneticmaterial may be bent or wound in order to produce a predetermined corestructure. Additionally, the assembled cores may be subjected tomachining or grinding, and, in some cases, etching with acids in orderto remove burrs and the like.

It has been proposed heretofore to treat the surfaces of ferrousmagnetic laminations with phosphoric acid in order to produce aninsulating iron phosphate film or coating on the surfaces of thelaminations. Extensive experience with phosphoric acid treated ferrousmagnetic material of this kind has shown that the insulation resistanceof the films is relatively low, often of the order of 1 to 2 ohms persquare centimeter when tested by simple contact pressure alone. When thetest contacts are applied with a force of 50 pounds per square inch uponthe phosphoric acid treated material and twisted, the insulationresistance frequently drops to values of the order of 0.15 to 0.5 ohmper square centimeter. Often the phosphate film is disrupted during thetwisting and the resistance is zero. For numerous applications, thelatter resistance values are too low to be satisfactory. The lack ofadherence and abrasion resistance as evidenced by decrease of resistanceupon twisting of the contacts as well as visual observation of thebreaking up of the film indicates that the prior iron phosphateinsulation is not satisfactory for many uses. Considerable difiicultyhas been encountered in actual experience with this type of insulation.

It has been suggested that the addition of powdered inert refractorysolids, such, for example, as magnesia, alumina, talc, or silica to thephosphoric acid might improve the insulating film on ferrous laminationstreated therewith. However, tests of the modified insulating film soproduced showed that the increase in resistance is relatively meager.Tests indicate that resistance values as low as from 1 to 5 ohms persquare centimeter are obtained with untwisted contacts, and theresistance values drop to about the same values as the plain ironphosphate films, when the contacts under pressure as above described aretwisted. The film therefore still has low strength and adherence.

The object of this invention is to provide a tenaciously adherentinsulating film possessing high ohmic resistance on ferrous magneticmaterial.

A further object of the invention is to provide for producing anadherent film having high ohmic resistance on ferrous magnetic materialby treating the material with a composition comprising phosphoric acidand an organic ester of silicic acid.

A further object of the invention is to provide a coating composition ofphosphoric acid and a silicon compound capable of decomposing to producemicroscropically fine silica for reaction with the phosphoric acidproducts when applied to ferrous material.

Other objects of the invention will, in part, be

obvious and will, in part, appear hereinafter For a better understandingof the nature and objects of the invention, reference should be had tothe followin detailed description and drawing, in which:

Figure 1 is a schematic view of a process for producing insulatedmagnetic strips and building a magnetic core in accordance with thisinvention;

Fig. 2 is a greatly enlarged fragmentary crosssectional view of theinsulated lamination of this invention applied thereto; and

Fig. 3 is a view in elevation, partly broken, of an insulation tester.

According to the present invention, insulating films of extraordinaryohmic insulation resistance which are tenaciously adherent and resistantto abrasion are produced on ferrous materials by applying to andchemically reacting therewith a solution having as its essentialingredients phosphoric acid and a silicon compound capable ondecomposition of producing microscopically fine silica. No insulatingmaterial known approaches either the resistance or adherence of the filmso produced.

The insulating film is produced by applying to ferrous material, such,for example, as silicon iron laminations, a coating of a solutioncomposed of a volatile solvent carrying substantial amounts ofphosphoric acid and a silicon compound capable of liberatingmicroscopically fine silica, such, for example, as tetraethyl silicate,and heat-treating the ferrous material and applied coating at atemperature of from 300 C. to 950 C. for a short period of time. Acomplex chemical reaction apparently takes place during the heattreatment. The phosphoric acid is believed to react with the ferrousmetal to produce iron phosphates. Simultaneously, the tetraethylsilicate decomposes to produce microscopically fine silica. There issubstantial evidence indicating that a chemical reaction between theiron phosphate (and perhaps the phosphoric acid) and the finely dividedsilica occurs. Either a true chemical action takes place or elsesecondary valence forces bonding the finely divided silica and ironphosphate are established, since unusual adherence and strength andother characteristics in the resulting film are secured not otherwiseattainable. While the foregoing facts appear to support our belief thata chemical reaction occurs, it is to be understood that this is onlyexplanatory and we are not to be held to this theory.

In preparing the coating composition for application to the ferrousmaterial, silicon compounds capable of producing on decomposition finelydivided silica are employed. Organic esters of silicic acid areparticularly desirable for this purpose. Typical esters are tetraethylsilicate, tetrabutyl silicate, methyl silicate, gylceryl silicate,diglycol silicate and ethylene glycol silicate, and condensed silicatessuch as hexa-ethyl sili cate, and mixtures of any of these. In thesubsequent examples and discussion, tetraethyl silicate will be employedas exemplary of other silicon compounds as well.

Sufllcient water should be present with the tetraethyl silicate or otherhydrolyzable silicates to produce a decomposition of the ester tosilicic acid which may then be readily converted to silica, in the formof silica gel, or the like.

Ortho-phosphoric acid has been found the most satisfactory substance foruse in the composition. Any strength phosphoric acid above about may beused. In practice 50% to 85% phosphoric acid is employed. In some cases,it has been found possible to substitute chromic acid up to an amountwhere it does not exceed 35% of the weight of the phosphoric acid.Furthermore, slight amounts of alkaline dihydrogen phosphates, such, forexample, as monosodium dihydrogen phosphate, may be substituted for asmall portion of the phosphoric acid. Considerations of cost and ease ofpreparation will render the ortho-phosphoric acid alone preferable inmost instances.

In preparing the composition, the phosphoric 1 acid and tetraethylsilicate are preferably made into a solution.. For this purpose it hasbeen found that either water or a simple aliphatic alcohol may beemployed as a solvent with good results. However, the best results wereobtained when a mixture of water and the alcohol were used as thesolvent for the phosphoric acid and the silicon compound. Examples ofsimple aliphatic alcohols suitable for the practice of this inventionare ethanol, propanol, isopropanol,

. ble that the simple alcohol be present in an amount equal to at leastabout half the weight of the water. It will be appreciated thatalcoholic solutions having substantial amounts of water, such, forexample, as 70% ethanol or 60% isopropanol may be used alone without theaddition of water. Denatured ethanol is satisfactory for use.

In preparing a solution, good results have been obtained by firstdissolving the tetraethyl silicate in the simple alcohol in thefollowing manner: 50 parts by weight of tetraethyl silicate and 30 partsby weight of 95% ethanol are mixed and permitted to stand about 12 hoursand then 10 parts of water are added. The tetraethyl silicate solutionso produced dissolves quite readily in the remainder of the solventalong with the phosphoric acid. Hereafter this solution will bedesignated in the examples as a tetraethyl silicate solution.

Reference should be had to the following examples, in which all partsare by weight, for the preparation and use of typical compositions inaccordance with the invention.

The water and ethanol were admixed and the phosphoric acid was thenpoured into the solvent mixture with stirring and finally the tetraethylsilicate solution was added. The solution was applied to silicon ironlaminations as a thin coating, and when heat treated at a temperature of450 C. for two minutes in a muille type furnace a hard, durable film wasproduced. When tested with the tester shown in Fig. 3 of the drawing,the insulating resistance was infinity. The insulation resistance didnot decrease appreciably even though the contacts were twisted throughan angle of while under a pressure of 50 pounds per square inch.

To show that a chemical reaction has taken place between the finelydivided silica and the iron phosphate in the film, a test was made onthe same silicon iron by applying a phosphoric acid solution similar toExample I, but having silica flour equal to the silica present in theethyl silicate; the film so produced has less than 2 ohms medianresistance with untwisted contacts. On twisting the film was disruptedand the resistance fell to about zero.

Referring to Fig. 1 of the drawing, there is illustrated a schematicdiagram of a suitable process for applying to ferrous magnetic materialthe composition of Example I, and building a wound core from theresulting insulated material. Strip silicon iron Hi from a supply coil 8is coated in a tank 12 containing the tetraethyl silicate-phosphoricacid solution H such as that of Example I. It will be understood thatthe composition H may be applied in other ways as by spraying.

flowing and the like. The strip I initially passes over a guide roll IO,thence into the solution where it passes under a submerged roller l8 andthen passes over two rubber squeeze rolls and 22 maintained under aselected pressure for predeterminin the amount of the solution l4 to berfatlained on the surfaces of the magnetic mater The strip l0 coatedwith the solution is then heat-treated in the furnace 24 at atemperature of from about 300 C. to 950 C. for a brief period of time.It is preferred to maintain a relatively low oxidizing or inertatmosphere in the furnace in order to prevent undue oxidation and theformation of rust. In an oxidizing atmosphere, such as air, heatingshould not be conducted for longer than five minutes at a temperature of550 C. or more than two minutes at 600 C. Good results have beenobtained in an open muille furnace by heating to a temperature of 600 C.for one minute. In a furnace provided with a relatively nonoxidizin oran inert atmosphere, for example nitrogen gas, the coatings may beheattreated at temperatures of 950 C. for 30 seconds as the upper limitand ten minutes or longer at temperatures of 300 C. with excellentresults.

The heat treatment applied to the strip l0 coated with the solution l4should be carried out in the light of the subsequent processing of theinsulated strip. If the insulated strip is to be reannealed, forexample, as is required to eliminate strains in wound cores producedtherefrom, the heat treatment in furnace 24 is preferably conducted attemperatures of the same order as the required subsequent reannealingtemperatures. In actual practice the reannealing temperatures for woundcores range from about 650 C. to 1000 C. and accordingly theheat-treating of the strip carrying the phosphoric acid and ester ofsilicic acid solutions should be conducted at temperatures approximatingthe reannealing temperature of cores prepared therefrom.

During the heat treatment in the furnace 24, the water and alcoholsolvent of the solution i4 evaporates, the phosphoric acid reacts withthe ferrous metal while the tetraethyl silicate decomposessimultaneously, whereby microscopically fine silica is precipitatedduring the reaction involving the phosphoric acid. The smoothest filmsare secured when the heat treatment is not too rapid so that vapors arenot evolved at too great a rate. A refractory, homogeneous filmcharacterized by good adherence and high electrical insulation isquickly produced.

As an example of the high adherence and mechanical strength of theinsulating film, the subsequent building of a wound core from theinsulated strip I0 is illustrated. Building of wound cores is believedto bethe most severe test of the properties of integral insulation onlaminations. The insulated-strip l0 passes through the guide rolls 28 toa winding machine where it is wound on a rectangular mandrel 28 into acontinuous rectangular or wound core 30. A pressure roll 22 is appliedto assure the formation of a tight core. It will be found that the stripmay be bent about the mandrel without the insulating film beingloosened, even though the radii at the corners of the mandrel 28 may be4; inch or less. The film does not powder or otherwise separate from thestrip during the winding operation.

The wound core 30 is then subjected to a strain anneal in an annealingoven 34 at temperatures of from 700 C. to 950 C. without the insulatingfilm on the laminations softening or fusing and sticking the laminationsto one another. This characteristic is quite important since fusedinsulation will give rise to strains when the core is cooled to roomtemperature. Insulating films produced by applying only phosphoric acidor phosphoric acid and an inert filler such as talc often results inadhesions being formed between laminations during the strain annealing,and it is necessary to apply physical force to separate the laminations.Application of the physical force is not only time-consuming anduneconomical, but may result in some damage to the core. The atmospherein the strain annealing oven is preferably one containing hydrogen.

In order to fill solidly the spaces between the turns, the core 30 afterstrain annealing is then impregnated in the tank 36 containing aninsulating resin 38. Thereafter the resin impregnated core 30 is bakedin the oven 40 to harden and polymerize the resin. While the core may beused in this condition, for many applications it must be cut, the resinbonded core 30 is then mounted in a jig 42 and severed intopredetermined sections by a cutter 44. It will be apparent that theresin bond strength cannot be better than the film adherence. It hasbeen found that abrasive bonded wheels, metal cutting saws, millingcutters, and other machine tools may be employed in machining the core30 to any desired shape and size without disruption of the insulatingfilm of this invention. Thereafter the cut core 30 is mounted on a jig46 for grinding the cut faces thereof by the grinding wheel 48 to renderthem smooth and plane. The ground core may be subjected to acid etchingin order to remove any burrs that may be present on the machinedsurfaces due to the grinding and machining operations.

The building of wound core sections is believed to be the severestpractical test of the adherence, heat and mechanical resistance, anddurability of an insulating film on magnetic laminations known to theart today. Thus the insulating film on the laminations must withstandextremely elevated temperatures and mechanical and chemical reagentswithout losing its adherence to the ferrous metal or deterioratingunduly in insulation resistance. In wound cores using oriented siliconiron, a minimum of one ohm per square centimeter is the least acceptableresistance, and preferably the resistance should be higher: 5 ohms persquare centimeter and better. The insulated laminations produced as heredisclosed have surpassed this requirement, Tests of completed cores showthat losses are extraordinarily low for laminations carrying the filmproduced from the phosphoric acid and silicon compound of thisinvention. Losses of considerably less than .1 watt per pound of 13 milsilicon iron at 15,000 gauss with 60 cycles alternating current areregularly obtained. Furthermore, high space factors of 96% to 99% aresecured. v

The insulated strip l0 produced bythe heattreating furnace 24 obviouslycan be made into any predetermined type of lamination by punching orcutting,vor otherwise machining to shape.

The insulating film will withstand punching operation such asencountered in producing L- shaped or other shaped laminations fortransformer cores and the like, ormotor and generator rotor and statorlaminationsin which sheets are punched into complex shapes. Theinsulating ed for vertical movement by cation of a predetermined tractedfor the passing of has from a value of 0.1 ohm By applying the tester toa sheet ofmag- 7 film will withstand the application and heat treatmentrequired for employing the usual baking varnishes and other insulatingmaterials employed in building motors, generators, transformers andother electricalapparatus.

A particularly desirable application of this invention is forpreferentially oriented silicon iron containing from 2% to 5%silicon-usually about 3.25% silicon. The preferentially oriented'silicon iron is extremely sensitive to strains and the insulating filmshere disclosed cooperate successfully therewith to produce substantiallystrain free insulated laminations. Not only strips but wires, bars,straps and the like can be treated.

'Other ferrous material, containing any of the usual alloying elements,aluminum, nickel, cobalt and the like, capable of reacting withphosphoric acid may be treated with the composition of this v inventionto produce the highly desirable electrical insulation disclosed. Sheetsilicon iron having up to 7% silicon, from '2 mils to mils in thickessis easily treated with the composition. The ferrous material may bethereafter formed to shape by winding, bending or otherwise mechanicallydeforming without anyserious impairment of the insulating film.

. Reference should be had to Fig. 2 of the drawing showing 'a greatlyenlarged cross-sectional view of the sheet magnetic material It with theadherent integral insulating film 50 on the surfaces thereof. The filmi0 is composed of iron phosphate chemically reacted with themicroscopically fine silica produced by the simultaneous decompositionof the tetraethyl silicate as described hereinbefore. In most cases, thethickness of the film 50 is approximately 2% of the thickness of thesheet l0. However, the thickness of the film 50 may be variedconsiderably from this ratio to suit individual requirements. On highlysatisfactory insulated 13 "mil sheet silicon iron, the film thicknesshas been measured as approximately 0.25 mil. However, in many cases thethickness has been varied 0.5 mil for 13-mil thick sheets and in allcases, the insulation resistance and adherence of the film has been at ahigh level.

Referring to Fig. 3 of the drawing, there is 11- iustrated an insulationtester 60, known as the v c-clamp spot tester, which has been found tobe extremely useful in evaluating'the properties magnetic material. The

of insulating films on a U-shaped frame 02 protester n is composed ofvided witha lower stationary contact member 64 movable contact 86guidand an upper relatively the shaft 80 passing 8 netic steel, 9,reading is obtained on the measuring instrument 84 when the contactssimply rest under the pressure of spring ll against the insulating'film,if'any thereon. By twisting the frame 02 with respect to the sheet beingtested, the insulating film between the contacts 64 and 60 is subjectedto relatively severe abrasion, and the resistance recorded on theinstrument .4 will generally drop considerably. No refractory materialknown on laminations has been able to withstand one twist ofapproximately-90 without a substantial drop in insulation resistance.Brittle or nonadherent films will be disrupted completely and theresistance will be zero. The insulatingmaterial of this invention willalmost invariably withstand three or four successive twists without theresistance dropping below the range of 10 to 100 ohms per squarecentimeter, and usually the insulationresistanee remains atapproximately infinity. Therefore it will be obvious that the nature ofthe insulation is entirely different, both in properties andperformance, over any magnetic sheet insulating material knownheretofore.

We have prepared numerous compositions in accordance with thisinvention, and have been able to produce therewith on magnetic materialinsulating films having the unexpected properties set forth. ,Theapplied compositions have been varied within the range of 100 parts ofsolvent, 5 to parts by weight of phosphoric acid, and from 1 to parts byweight of tetraethyl silicate or its equivalent. Since the organicsilicates vary a silicon dioxide equal to from 1% to 100% of the 1weight of the phosphoric way, the compositions comprise 30partsbyweight' from 0.1 to

through the upper part of the frame 02. A spring 1| encircling the shaft6| provides for the applipressure of 50 pounds per'square inch at theface of the contact. A manually operated lever 12 connected to the shaft08 enables the upper contact 06' to be rea sheet between the contacts 84and 66. An insulated conductor."

connects the movable contact 68 in circuit with" an insulated bushing 16while a second insulated conductor 18 connects the stationary contact llto a second insulated bushing 80. Two conductors 02 connect the bushings16 and 80 to a meter device 84 containing a source of low voltagecurrent for measuring the resistance between the contacts 64 and 86.-The measuring device 84 is capable of determining'ohmic resistance ofcoatper square centimeter up to infinity on .a scale.

acid. Stated another of phosphoric acid, an organic ester of silicicacid in an amount sufiicient to produce on decomposition silicondioxide-in an amount equaltofrom 1 to 100% of the weight of thephosphoric acid, and from 50 to 400 parts by weight of a solvent for thephosphoric acid and the silicic 'acid ester, the solvent containingsufiicient water to hydrolyze the silicic acidester. Outstanding resultsare obtained by'employing suflicient organic silicate ester to providesilica equal to from 4% to 20% of the weight of the phosphoric acid.

In applying the compositions to silicon iron, ood results have beenposition has been applied in the amount of from 1 to 10 gallons per tonof 13-mil thick sheets of silicon iron. tained with the application ofthree ton of the composition of Example I.

The following examples list various satisfactory formulations of thecomposition with which good results were obtained when applied toferrous material.

Example I! a Parts 95% ethanol 80 Water 120 Phosphoric acid 60Tetraethyl orthosilicate solution --e 20 trample III 95% ethanol 50Distilled water H3P04 i. 80

'75v Ethyl silicate solution 30 obtained when the com- Excellent resultshave been ob- Example 17 Parts 95% ethanol 50 Distilled water 50 HaPO4(85%) 60 (210: Ethyl silicate solution so Example V Parts 95% ethanol100 HsPO4 80 Ethyl silicate solution 70 Example V1 Parts 95% ethanol 50Water 50 11:104 l Ethyl silicate solution. 30 C313: 1 NaHzPO4 10 ExampleVII Parts Isopropyl alcohol (99%) 50 Water 50 Ethyl silicate solution 30Phosphoric acid '10 Example VIII Parts 95% ethanol 100 Fhosphoric acid,85% so Ethylene glycol silicate 3o Example 1X Parts 95% ethanol 1Q!)Phosphoric acid, 85% l0 Ethylene glycol silicate so 'Za'a'mpZe X PartsEsopropanol, 99% iii!) Phosphoric acid, 85% 80 Ethylene glycol silicate20 Example X11 Parts t"5% etha 50 Water 50 Phosphoric acid, 85% 7cEthylene glycol silicate solution 30 Example XII Parts 95% ethanol 100Phosphoric acid, 85% 70 Ethylene glycol silicate 30 ClzOa 5 Example XIIIParts Distilled water 100 Phosphoric acid, 85% '10 Ethylene glycolsilicate 30 In the above examples, it has been found that ethyleneglycol silicate produces somewhat smoother films than the tetraethylsilicate. In all cases the insulating films had high resistance andadhered well to the magnetic sheets.

We have found that the inclusion of solid fillers in the compositions ofthis invention is not particularly beneficial. For example, finelypulverized magnesium oxide, aluminum oxide or silica flour appeared toreduce the insulation resistance and adherence of the films. While small10 rated if desired for certain purposes, not advised.

While it has been proposed to employ an organic silicate. such astetraethyl silicate, alone for producing a siliceous coating on surfacesof magnetic laminations, tests of such materials with the tester of Fig.3 of the drawing indicate that the ohmic resistance and the adherenceare relatively poor. The decomposition of ethyl silicate, alone, or withfinely powdered aluminum oxide filler for instance, produces a very thinfilm that lacks both ohmic resistance and adhesion. The ohmic resistanceat best is much less than one ohm per square centimeter and the twistingof the c-clamp tester disrupts the film and the resistance drops tozero. Therefore, it is totally unexpected that the combination of theiruse is phosphoric acid with an organic esterof $111010 acid will producean insulated coating having such extraordinary adherence and ohmicresistance as disclosed herein. Numerous other advantages obtained bythe combinations set forth are obtained that are not available withprior art insulating coatings from magnetic material.

Since certain changes in carrying out the above process and certainmodifications in the article which embody the invention may be madewithout departing from its scope, it is intended that all mattercontained in the above description or shown in the accompanying drawingshall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. In the process of treating ferrous metal sheets to provide on thesurfaces thereof a tenaciously adherent film possessing high ohmicresistance, the steps comprising applying to the sheets a coatingcomposition composed of 30 parts by weight of phosphoric acid, anorganic ester of silicic acid in an amount sufilcient on decompositionto produce silicon dioxide equal to from 1% to of the weight of thephosphoric acid, and about from 50 to 400 parts of a solvent for thephosphoric acid and organic ester of silicic acid, the solvent composedof water and a simple compatible aliphatic alcohol, the alcohol being atleast half the weight of the water, the water being present in an amountsuflicient to hydrolyze the organic ester, the composition applied in anamount of from 1 to 10 gallons per ton of 13 mil thick sheet, andheat-treating the ferrous metal sheets and applied composition at atemperature of from about 300 C. to 950 C. to react the phosphoric acidwith the ferrous metal, to liberate microscopically fine silica bydecomposition of the organic ester of silicic acid, and to volatilizethe solvent, the reaction product "of the phosphoric acid and theferrous metal and the fine silica producing the adherent insulatingcoating.

2. In the process of treating ferrous metal sheets to provide on thesurfaces thereof a tenaciously adherent film possessing high ohmicresistance, the steps comprising applying to the surfaces of the sheetsa coating composition composed of from 5 to 60 parts by weight ofphosphoric acid, from about 1 to 65 parts by weight of the tetraethylsilicate, and 100 parts by weight of a solvent, the solvent composed ofwater and a compatible simple aliphatic alcohol, the alcohol being atleast half the weight of the water, the water being present in an amountsufflcient to hydrolyze the tetraethyl silicate, the composition beingapplied in an amount of from 1 to 10 amounts of solid refractories maybe inc rpo gallons per ton of 13 mil thick sheets, and heattreating theferrous metal sheets and applied composition at a temperature of from300 C. to 950 c.to react the phosphoric acid with the ferrous metal, toliberate microscopically fine silica by decomposition of the tetraethylsilicate, and to volatilize the solvent, the reaction product of thephosphoric acid and the ferrous metal and the fine silica producing theadherent insulating coating. T

3. In the process of treating ferrous metal sheets to provide on thesurfaces thereof a tenaciously adherent fllm possessing high ohmicresistance, the steps comprising applying to the surfaces of the sheetsa coating composition composed of from 5 to 60 parts by weight ofphosphoric acid, from about 1 to 65 parts by weight of the ethyleneglycol silicate, and 100 parts by weight of a solvent, the solventcomposed of water and a, compatible simple aliphatic alcohol, thealcohol being at least half the weight of the water, the water beingpresent in an amount sumcient to hydrolyze the ethylene glycol silicate,the composition being applied in an amount of from 1 to gallons per tonof 13 mil thick sheets,'and heat-treating the ferrous metal sheets andapplied composition at a temperature of from 300 C. to 950 C. to reactthe phosphoric acid with the ferrous metal, to liberate microscopicallyfine silica by decomposition of the ethylene glycol sillcate, and tovolatilize the solvent, the reaction product of the phosphoric acid andthe ferrous metal and the fine silica producing the adherent insulatingcoating.

4. In the process of providing a tenaciously adherent film possessinghigh ohmic resistance on a ferrous metal sheet, the steps comprisingapplying to the surfaces of the sheet a coating composition composed ofEtc 60 parts by weight of a organic ester, but not mixture of phosphoricacid and chromic acid, the chromic acid not exceeding 35% 01 the weightof the phosphoric acid, from 1 to parts by weight of an organic ester ofsilicic acid, parts by weight of a solvent composed of water and acompatible simple aliphatic alcohol, the water being present in anamount sufliclent to hydrolyze the exceeding half the weight of thesolvent, and heat-treating the sheet and the applied coating compositionto provide for simultaneously reacting the phosphoric and chromic acidswith the ferrous metal and liberating microscopically fine silica fromthe organic ester of silicic acid.

FRITZ J. NAGEL.

CLIFFORD C. HORSTMAN.

REFERENCES CITED The following references are of record in the file ofthis .patent:

UNITED STATES PATENTS Number Name Date 1,750,270 Jones Mar. 11, 19301,889,654 Griessbach et a]. Nov. 29, 1932 2,144,425 Cook Jan. 17, 1939.2,161,319 Schamberger June 6, 1939 2,413,949 Broverman Jan. 7, 1947FOREIGN PATENTS Number Country Date 679,083 France Jan. 5, 1930 570,990Germany Feb. 22, 1933 OTHER REFERENCES King: "Paint, Varnish, Lacquer,Enamel and Colour Manufacture," May 1931, pages 52 to 55.

